The present invention relates to tracking circuits, and more particularly to a circuit capable of tracking a signal returned from a moving object as the signal passes through antenna nulls.
It is well known that radar and other electronic direction finding systems make use of the physical property that a transmitted radio frequency signal is reflected by surrounding objects. One important application for such systems is object tracking. An object tracking system's primary purposes are to determine the position and speed of a moving object by monitoring the reflected radio frequency signal as received by an antenna. The object's position is determined by using a directional receiving antenna. Such directional antennas are characterized by an antenna pattern consisting of a narrow or highly focused main lobe and attendant sidelobes. The main lobe is steered towards the moving object by feeding the antenna's electrical output to a control circuit. The control circuit controls a motor arranged for rotating the antenna. When the electrical output is at a maximum, the antenna is known to be pointing at the moving object. If the electrical output's amplitude decreases, the control circuit uses a resulting phase change in the electrical output to activate the motor so the antenna will be realigned. The object's speed is determined by using the physical property that a reflected radio frequency signal returned from a moving object exhibits frequency shift. This frequency shift, or Doppler frequency, can be detected in the electrical output to determine the object's speed.
A problem occurs with Doppler frequency tracking systems when the object moves too quickly for the antenna to be realigned, thus allowing the reflected radio frequency signal to be received through a sidelobe rather than through the main lobe. This causes an abrupt change in the antenna's electrical output as the reflected signal moves through the antenna nulls. One way of solving this problem is to use a signal processing circuit to filter abrupt changes in the electrical output. Thus, the rotation direction and Doppler frequency as determined before the abrupt amplitude or phase change occurred is used until the antenna is again aligned with the moving object. Although this technique works well, for some applications there is a period of time when the moving object is not being directly tracked. Also, Doppler frequency information may be lost by the filtering process.
Another solution to the object tracking problem is to use multiple antenna elements arranged as a phased array and beamforming techniques on outputs of the phased array to develop a plurality of steerable antenna lobes in a plurality of directions. Such systems work well for their intended purpose, but tend to be more expensive than single antenna systems.
A somewhat analogous problem is that of recovering data in a phase modulated communication signal. As is well known, a phase modulated communication signal is one where the phase of a carrier signal is modulated in accordance with data to be transmitted. In one such technique, known as biphase modulation, the communication signal consists of a carrier signal at a particular frequency transmitted with a particular phase, for example 0.degree., to indicate a logic 0 data bit, and with another phase, for example 180.degree., to indicate a logic 1 data bit. The receiver then reconstructs the data by demodulating the phase shifts in the communication signal. This demodulation problem is analogous to the object tracking problem in that they both process signals containing abrupt phase shifts. An example of one such system is shown in the United States patent granted to DeLong et al., U.S. Pat. No. 3,883,806, issued May 13, 1975 and assigned to Rockwell International Corporation of Dallas, Texas. This circuit provides a local reference signal remaining phase-continuous despite phase modulations in an input signal. The circuit operates by reversing the output of a phase lock loop each time the input signal reverses phase. However, such a circuit is not directly applicable to the present problem since the purpose of such a circuit is to detect and remove such abrupt phase transitions, whereas in the present invention it is important not only to detect and remove transistions, but also to provide a continuous and smooth indication of the instantaneous phase of the reflected signal for tracking purposes.